1. Field of the Invention
The present invention generally relates to a guideway for a magnetic levitation transportation system and more particularly, to a new and improved expansion joint for such a guideway that minimizes transients in magnetic lift and drag forces on a magnetic levitation vehicle as the vehicle travels over the joint.
2. Background of the Invention
A considerable amount of research has been devoted to magnetic levitation transportation systems, particularly for public transportation in countries having densely populated regions. The interest in such magnetic levitation transportation systems are due in part to such factors as energy conservation, high speed travel at ground level, and economic and environmental problems associated with conventional systems. In such a system, a magnetic levitation vehicle is suspended above a guideway by the interaction of superconducting magnets with the guideway such that no physical contact occurs with the rail or roadbed once the vehicle is in motion. The velocity that can be attained by such vehicles is partly dependent on the magnetic lift and drag forces produced by the superconducting magnets interacting with the guideway over which the vehicle is to travel.
Short-circuited conducting loops or a continuous conducting guideway in which large eddy currents are induced by the superconducting magnets mounted on the magnetic levitation vehicle have been proposed for such systems. One advantage of the continuous guideway design is that it avoids the periodic variations in the lift and drag forces that are encountered when loop guideways are used. A continuous track or guideway nevertheless needs to be designed to allow for the difference in the coefficient of thermal expansion of the material from which the guideway is made and the base on which the guideway is supported. For example, an aluminum guideway may be typically mounted on a reinforced concrete base, but the coefficient of thermal expansion of the aluminum will tend to exceed the coefficient of thermal expansion of the concrete. While different aluminum alloys may be used to increase the strength of the guideway and decrease the difference in the coefficient of thermal expansion of the guideway and the base on which it is supported, such alloys tend to have the disadvantage of having a reduced conductivity.
Consequently, discontinuities or expansion joints need to be provided in the guideway to allow for such expansion and contraction. However, the interruption to the flow of induced eddy currents at such discontinuities will cause force perturbations at the joints. For example, truncation of the eddy currents at the discontinuity will cause a loss in the lift forces and the magnetic drag force will vary due to a change in the power dissipation in the guideway resulting from a modification of the eddy currents close to the discontinuity.
A comprehensive study has been made of the effects of different configured guideway joints on these types of forces. The study is reported by Atherton et al. in "Joints in Strips for Electrodynamic Magnetic Levitation Systems", IEEE Transactions on Magnetics, Vol. MAG-14, No. 2, pp. 69-75 (March 1978). The report describes seven different designs for guideway joints that were considered: (1) simple butt joint, (2) angled butt joint, (3) strapped joint, (4) joint with flanged ends, (5) overlap joint, (6) butt joint with a backing strip and (7) butt joint with a covering strip. Actual force measurements on a simple butt joint and a butt joint with a backing strip were reported and in addition, force estimates for all of the joints were reported using an impedance modeling technique. As reported by Atherton et al., those joint configurations all resulted in considerable transients in both the lift and drag forces.
In a report by R. H. Borcherts, "Repulsion Magnetic Suspension Research--U.S. Progress to Date", Cryogenics, pp. 385-393 (Jul. 1985), variations in lift and drag forces caused by joints in guideways for a magnetic levitation transportation system were analyzed using an impedance modeling technique. The same conclusion was reached, namely, the evaluated joints will cause transients in the lift and drag forces so as to effect the ride of a magnetic levitation vehicle traveling over the joints. Other reports of the effects of joints in guideways for magnetic levitation transportation systems include Coffey et al., "The Feasibility of Magnetically Levitating High Speed Ground Vehicles", U.S. DOT Report, FRA-RT. 72-39 (Feb. 1972) and Marin et al., "Forces on a Line Current Moving Above a Discontinuous Ground Plane", Journal of Applied Physics, Vol. 45, No. 5, pp. 2055-2057 (May 1974).
These studies show that undesirable transients will occur with respect to the lift and drag forces as a magnetic levitation vehicle passes over the different configured joints that have been proposed for use in a guideway for a magnetic levitation transportation system. However, these studies do not indicate how those undesirable transients in the lift and drag forces can be reduced in order to provide an improved magnetic levitation transportation system that will among other things provide a smoother ride for passengers in magnetic levitation vehicles used in the system.